The invention concerns a method for manufacturing a bristle structure on an arbitrary kind of carrier using a molding tool having channels, extending from a mold wall, for the molding of bristles, wherein a mold mass for forming the bristles is introduced into the channels under pressure.
Bristle products which include, in particular, brushes of every kind, paint brushes as well as brooms are predominantly produced using mechanical methods with which the bristle carrier is initially made having holes and the bristle bundles are mechanically inserted at a subsequent time. With the advent of plastics the brush bodies are manufactured through injection molding or molding processes and the bristles are attached either in the conventional mechanical fashion or, recently, using thermal techniques. In all such cases the bristles monofilaments must initially be produced using an extrusion or a spinning process with the monofilaments being optionally cut and the bristles subsequently attached to the carrier. The anchor technique, which is predominantly used in current processes, entails looping the bristles and punching the bristles, together with an anchor made from metal, into the brush body.
For these reasons there have been a plurality of attempts to render production processes more efficient by shaping the bristles or bundles together with a carrier as a single piece and to subsequently bond the carrier to the brush body. More than 100 years ago (GB 788/1861, GB 24935/1896) individuals have proposed molding the bristles along with a carrier binding same from an elastic material such as rubber or the like and subsequently attaching the structure to a rigid brush body. DE 941364, GB 2,151,971, U.S. Pat. No. 3,016,44, U.S. Pat. No. 4,244,076, U.S. Pat. No. 5,040,260, U.S. Pat. No. 5,966,771 and WO 98/03097 propose the production of cleaning elements bundled into groups together with a carrier binding same in an injection molding process and binding the carrier to a brush body at a subsequent time or using a two-component injection mold. U.S. Pat. No. 5,926,900 proposes the manufacture of the entire brush body together with the bristles as an integral injection molded member.
Bristles of this type can, for practical reasons, only be utilized in limited applications e.g. for hair care or as throwaway brushes. The reason for the deficient applicability and lack of acceptance is the fact that bristles which are injection molded have a fatigue strength under reverse bending stresses which is inadequate because, in contrast to bristles which are extracted from a spinning process, they do not have the molecular structure which is necessary for stability and which is primarily distinguished by a longitudinal orientation of the molecular chains, parallel to the bristle. For this reason they are most appropriately designated as working or cleaning elements than as bristles. Inadequate stability is in particular present in the shoulder region of the working elements proximate the carrier since an orientation of the molecules is nearly completely absent at this location. In consequence thereof, the working elements, which are initially properly oriented directly following manufacture, change their position after short periods of use, in particular through bending, kinking, or failure to return to an upright position (bend recovery). Moreover, these techniques require use of one and the same plastic for the working elements and for the carrier, which, for high quality plastics from which bristles having high quality requirements must be made, leads to corresponding increase in costs. Attempts to reduce costs require a compromise in the choice of plastic. Nevertheless, the substantial disadvantages in applications remain in consequence of which these types of brushes are only suitable for limited applications. Moreover, there is no possibility for adaptation of the material used for the carrier and the brushes in response to individual requirements or for differences among the bristles, in particular with regard to mechanical stability, material specific coefficients of friction, color etc.
U.S. Pat. No. 2,621,639 discloses brushes with which the working elements (pins) are not bristles in the traditional sense rather pins, rods, strips or the like. These working elements are injection-molded elements, which are predominantly made from rubber or rubber elastic plastics such as elastomers and have large cross-sections and also shorter lengths than most bristles. These structural restrictions for the “bristles” are dictated by two facts. First of all, only in this manner can a moderately satisfactory stability and fatigue strength under reversed stresses be achieved. Second of all, for injection molding technical reasons, the molding channels cannot be sufficiently narrow and deep in order to guarantee proper filling of the mold and as well as to facilitate removal from the mold. The characteristic feature of these “pins” is substantially a soft action on the surface that is treated by the elements with an increased coefficient of friction to effect a kind of rubbing or massaging effect but not an active brushing action. Typical applications are hair brushes which are predominantly used for separation and ordering of the hair and which are intended to only massage the scalp. The stiffness of these elements can only be substantially influenced by the diameter, the ratio of the diameter to the length, and by the hardness of the plastics used.
The brushes according to U.S. Pat. No. 2,621,639 are produced by injection molding. A flexible thin carrier plate having perforations corresponding to the configuration of the working elements is introduced into an injection mold having a plurality of channel-like molding cavities which are adjacent to the perforation holes in the carrier and which serve to shape the pin shaped working elements. Distributor channels are disposed on the opposite side (injection side) and serve to pass the liquid plastic melt, e.g. nylon, to the individual perforation holes and into the adjacent molding channels. The molding channels initially have a widening in direct proximity to the perforations in the carrier. In this manner, a bulge is produced on both sides of the thin carrier plate so that the working elements are axially secured in both directions. Despite the fact that nylon would be theoretically appropriate to produce bristle-like properties, this does not occur, since a longitudinally oriented molecular structure cannot be effected, at least at the foot of the cleaning element due to the bulge. A similar situation obtains for another conventional hair brush (EP-B1-0120229) with which a carrier with bushing shaped conical shoulders is initially injected and an additional plastic is subsequently injected into the bushings to form a core and seats with an enlargement at the opening mouth of the bushing. The cores are bound together at the back side with a second carrier plate made from an appropriate material to effect a positive, axially secure connection between both components as the principle goal. As a result, the working elements become even more bulky.
Such cleaning elements have also been proposed for tooth brushes and for brooms (U.S. Pat. No. 5,040,260, U.S. Pat. No. 5,966,771). These brushes are structured from two components. U.S. Pat. No. 1,924,152, U.S. Pat. No. 2,139,242, DE 826440, and WO 0/64307 have proposed combining a bristle stock made from conventional bristles and having their recognized and cleaning properties with rod or pin-like cleaning elements made from rubber elastic plastic.
WO/03831 (DE 10033256 and DE 10130863), assigned to the same party as the instant application and the complete disclosure of which are hereby incorporated by reference, describe a procedure and a device with which the well-known advantages of injection molded technology can be utilized. Nevertheless, the production process is not suitable for bristle products having bristles of quality and application properties that are comparable to those of spin extruded bristles and certainly not better or improved with respect thereto. The carrier is thereby produced having openings in the form of spinning nozzles, wherein the openings, to which the channels are adjacent, have a minimal width of less than or equal to 3 mm, at least over a portion of their length, wherein the ratio between this width and the flow path of the melt given by the combined height of the openings and the length of the channels is chosen to be less than or equal to 1:5, with the plastic melt being injected from at least one side of the carrier (the introductory side of the melt) and through the openings to form the bristles in the channels. The above mentioned ratio is preferentially chosen to be less than or equal to 1:10. The lower limit of this ratio can lie in the range of 1:250.
This latter procedure constitutes an entirely new approach for manufacturing bristle products. The carrier on which the bristles are disposed which can constitute the brush body itself or a portion thereof, e.g. in the form of an insert or the like, simultaneously serves as a lost or incorporated “tool” for the production of the bristles by injection molding. Blockage effects and wall friction lead to an extensional flow having relatively large sheer forces proximate the walls at the “spinning nozzle” openings. This causes the molecular structure within the mold to become oriented in the flow direction with this orientation continuing into the channels forming the bristles, wherein the ratio between the flow path length for the melt chosen in accordance with that invention and the size of the most narrow region of the openings, optimizes the longitudinal molecular orientation. This self-reinforcement of the bristles in response to longitudinal orientation of the molecular chains is particularly apparent for partial crystal thermoplastics. Moreover, bristle products produced in this fashion have, in contrast to bristles produced in an integral fashion, a short partial length of the bristle (the root) disposed within the carrier and supported thereby. This root region is the most sensitive portion with respect to strength since the orientation of the molecules at this location is inadequate. This stabilization leads to a higher fatigue strength under bending stresses, in particular in response to reverse bending stresses, in additional to a higher tensile strength. Compared to conventional one piece injected bristle products, the above mentioned factors provide for an increase of 40% or more in the bending forces which are necessary to procure a given deflection of the bristle. The module of elasticity is also increased substantially. Since the tensile strength is also substantially increased, these bristles can be easily removed from the mold, even when they have narrow cross-sections and large lengths.
The conventional procedure in accordance with WO 02/03831, assigned to the assignee of the instant invention, can be used, in principle, with all flowable plastics, wherein in view of the requirements for bristles, thermoplastics or thermo elastics or mixtures thereof (alloys) are preferably utilized, since such plastics allow for the most prominent degree of molecular orientation.
The previously mentioned procedure, due to the assignee, as well as the procedure in accordance with U.S. Pat. No. 2,621,369 have the disadvantage that the hole pattern in the prefabricated plastic carrier must identically match the predetermined grid defined by the channels in the molding tool. In particular, the axes of the openings and the channels must be aligned. In the event that this requirement is not precisely maintained, shifts occur in the region of the brush anchoring leading to an undesirable decrease in the cross-section at the foot of the bristle so that the bristle tears off or sheers off prematurely. On the other hand, the bristle carrier having the predetermined holes and made from plastic can only be produced in a relatively economical fashion using the injection molding procedure. However, injection molding nearly invariably leads to a shrinkage in the carrier not only due to the cooling process but also due to post-shrinkage over a subsequent extended period of time as a result of which the initial geometry is lost. Moreover, one is restrained to use of low shrinkage plastics and is therefore substantially restricted with respect to the material combinations for the bristles and carrier. If one desires to facilitate an increased range of material for the bristle carrier combination and, in particular, for the material of the carrier, it may be necessary to provide for a plurality of injection molding tools.
It is the underlying purpose of the present invention to propose a method with which a reproducible bristle structure of constant quality can be produced using arbitrary carrier materials.